The disclosed embodiments generally relate to the production of 2-methyl-2-pentene and isoprene and more particularly to the use of a mixed C4 stream to produce 2-methyl-2-pentene, which subsequently is converted to isoprene.
2-methyl-2-pentene is a product found in steam cracking effluents at low concentrations. It is in the presence of many closely boiling C5 and C6 olefin isomers, making recovery at significant volumes difficult and expensive.
It is known from U.S. Pat. No. 6,538,168 to produce C5/C6 olefins from a stream of C4 hydrocarbons. The initial metathesis reaction of the C4 olefins produces a mixture of C2-C6 olefins and butanes. In one embodiment described in U.S. Pat. No. 6,538,168, the C2-C6 olefins mixture is fractionated in a multi-stage distillation process to give a low boiling fraction A containing C2-C4 olefins and butanes, or C2-C3 olefins, a higher but still low boiling fraction B containing butenes and butanes, an intermediate boiling fraction C containing pentene and methylbutene, and a high boiling fraction D containing hexene and methylpentene. In some cases, the C5 and C6 olefins are separated from one another by fractionation. In Example 3 described therein, the product stream contains 99.5% wt. 3-hexene. The 1-butene conversion rate is 91% wt. and the 2-butene conversion rate is 50% wt. Isobutylene is removed before metathesis so that the reactant stream for metathesis contains only 2.0% isobutylene. The concentration of 2 methyl 2-pentene in this case is low (<1%).
Other known processes produce 2-methyl-2-pentene from propylene. A process that involves dimerizing propylene to form methyl-pentenes and dimethyl-butenes is described in U.S. Pat. No. 3,686,352. However, when 2-methyl-2-pentene is formed using propylene as a reactant, feedstock costs are high.
Isoprene (2-methyl-1,3-butadiene) is a precursor to 1,4-polyisoprene, which is a synthetic version of natural rubber. Isoprene is conventionally recovered from the C5 fraction of the effluent of steam cracking of naphtha or heavier oils. The steam cracker C5 stream is sent to a solvent extraction process typically using acrylonitrile as the solvent. This route is complicated by the presence of cyclopentadiene that is also removed selectively by the solvent.
In the 1950's and 1960's, the Goodyear Tire and Rubber Company developed a process for converting 2-methyl-2-pentene to a range of hydrocarbons, mainly isoprene, methane, and isobutylene. The 2-methyl-2-pentene can be subsequently cracked in a pyrolysis reactor to form isoprene and other products, as is described in “Factors Affecting Methyl Pentene Pyrolysis”, Frech et al, ACS Symposium Series, 1976) The products comprise methane, ethylene, butadiene, isoprene butenes, and C6 dienes. The reaction takes place at temperatures of 600 Deg. C. or higher and under high dilution to avoid isoprene polymerization. The Goodyear process used HBr or a mixture of H2S and NH3 as a homogeneous catalyst to promote selectivity of isoprene in a thermal cracking reaction. Goodyear tests with HBr showed single pass yields of 54.5% mol. isoprene from 2-methyl-2-pentene. The isoprene then can be separated from this mixture by using extractive distillation. This work is discussed in Lloyd M. Elkin, Isoprene, Stanford Research Institute Process Economics Report No. 28 (p. 60), 1967.
An alternate isoprene production route is to dehydrogenate isoamylenes (methyl butenes). The dehydrogenation step has been practiced commercially by Air Products using their Catofin process. Isoamylenes typically have been recovered by steam cracking C5 product streams. In those cases however, the process is complicated by the presence of cyclopentadiene. The C5 product stream requires selective hydrogenation to remove the dienes that would rapidly foul a dehydrogenation catalyst. The dehydrogenation step is also very costly, requiring vacuum operation and significant capital expense. Isoamylenes can also be formed by the metathesis of butenes and propylene, as is described in Ind. & Eng. Chemistry Prod. Res. Develop., Vol. 10, No. 1, 1071 pg. 46). These isoamylenes can then be dehydrogenated to isoprene as above.
It would be useful to develop a process that results in a high yield of 2-methyl-2-pentene from mixed C4 olefin streams, combined with high butenes conversion. The 2-methyl-2-pentene can then be used to produce isoprene.